Dementia in Parkinson's disease (PD) patients is often associated with pathological changes in ?-synuclein, Amyloid (A) and tau proteins. In our new preliminary study, the CA1 and CA3 regions of the hippocampus are highly vulnerable to ?-synuclein pathology as well as tau deposits in the brains from PD patients. Clarifying the cellular interaction between ? synuclein and tau will help better understand the pathogenesis of PD dementia. Interestingly, in our preliminary studies, the expression of PD-related A53T mutant a-synuclein proteins drives tau proteins to dendritic spines, causes loss of AMPA receptors in spines and results in memory loss. To find a postsynaptic link between ?-synuclein and tau, we will test the central hypothesis that A53T ?-synuclein impairs post-synaptic structures and/or functions by causing tau mislocalization to dendritic spines, which subsequently leads to calcineurin- and GluR1 S845 dephosphorylation-dependent AMPA receptor internalization. The central hypothesis will be tested by two specific aims: In Specific Aim 1, we will perform biochemical, live imaging, immunocytochemical and electrophysiological experiments to determine the role of tau mislocalization and/or tau hyperphosphorylation in postsynaptic dysfunction caused by ?-synuclein. We will test the hypothesis that ?-synuclein induces synaptic dysfunction by causing loss of AMPA receptor through tau mislocalization and tau hyperphosphorylation. In Specific Aim 2, we will use pharmacological, molecular biological, genetic, imaging and electrophysiological approaches to determine the roles of calcineurin and tau in ?-synuclein-induced AMPA receptor internalization. We will test the hypothesis that ?-synuclein induces AMPA receptor internalization via tau- and calcineurin-dependent dephosphorylation of Serine 845 at the C-terminus of GluR1 subunits. The completion of the proposed project will unravel a new final postsynaptic cascade that leads to functional deficits after a-synuclein initiation, which may occur at either a pre- or post-synaptic location. The unraveled signaling steps may be targeted in future AD and PD therapeutic strategies.